An RF power amplifier provides the final stage of amplification for a communication signal that has been modulated and converted into an RF signal. Often that RF signal exhibits frequencies in a predetermined RF frequency band licensed by a regulatory agency for a particular use. The RF power amplifier boosts the power of this RF communication signal to a level sufficient so that the signal, when it propagates to an antenna, will be broadcast in such a manner that it will meet the communication goals of the RF transmitter.
Many popular modern modulation techniques, such as CDMA, QAM, OFDM, and the like, require the RF power amplifier to perform a linear amplification operation. In other words, the RF communication signal conveys both amplitude and phase information, and the RF power amplifier should faithfully reproduce both the amplitude and phase content of the RF signal presented to it. While perfect linearity is a goal for any linear RF power amplifier, all linear RF power amplifiers invariably fail to meet it. The degree to which the goal of perfect linearity is missed leads to unwanted intermodulation, distortion, and spectral regrowth. Spectral regrowth refers to an expansion of the bandwidth of an RF communication signal. Governmental regulatory agencies define spectral masks which impose stringent constraints on the spectral emissions from transmitters. Even small amounts of spectral regrowth can cause the transmitter to violate regulatory requirements.
Those who design RF transmitters understand that different RF power amplifier limitations lead, directly and indirectly, to different types of nonlinearities. One of these nonlinearities results indirectly from an unwanted amplifier-generated low-frequency distortion signal referred to as a video signal. In particular, RF power amplifiers tend to generate unwanted harmonics of the fundamental RF communication signal being amplified along with the desired amplified fundamental RF communication signal. Filters are often used to remove or otherwise block the harmonics from being broadcast from the transmitter. But the even harmonics have sub-RF byproducts below the fundamental in frequency, extending upward from zero Hz. In some RF power amplifiers, the even harmonics and their sub-RF byproducts may be less prominent, but other mechanisms are present to generate other forms of sub-RF distortion. This sub-RF, amplifier-generated distortion signal represents the sub-RF energy that extends upward from zero Hz. While the amplifier-generated sub-RF distortion signal is not broadcast from the transmitter, it may nevertheless cause problems.
A typical RF amplifier uses an RF amplifying device which is fed a biasing voltage through a biasing network. The sub-RF distortion signal causes a time-varying voltage to develop across the biasing network, which causes a corresponding and unwanted time-varying voltage modulation of the bias voltage applied across conduction nodes of the RF amplifying device. This unwanted bias modulation leads to an unwanted intermodulation between the sub-RF distortion signal and the RF fundamental signal. The intermodulation causes the RF power amplifier to generate an RF distortion signal which resides in the bandwidth of the fundamental RF signal and extends outside the bandwidth of the fundamental RF signal. This type of distortion is undesirable because it reduces the signal-to-noise ratio of the transmitted RF signal. But it is highly undesirable due to the spectral regrowth which often must be corrected in order for the transmitter to comply with its spectral mask. Thus, the sub-RF distortion signal causes the RF amplifying device's bias signal to be less stable than desired. Without this sub-RF distortion signal form of bias corruption, linearity would improve.
Conventional transmitters have addressed the sub-RF distortion signal problem in at least a couple of different ways. In one way, the biasing network is configured to implement a series of resonant impedance notches distributed throughout a bandwidth of the sub-RF distortion signal. This technique lowers the overall impedance of the bias network in the sub-RF distortion signal bandwidth, which in turn attenuates or otherwise somewhat stabilizes the sub-RF distortion signal and reduces the unwanted intermodulation. Unfortunately, this technique does not work well for wide bandwidth signals. One of the requirements of a bias network is to exhibit very high impedance to the amplified fundamental RF signal. For wide bandwidth communication signals it becomes increasingly difficult to configure a biasing network to exhibit adequately low impedance throughout a wide sub-RF distortion signal bandwidth yet exhibit adequately high impedance at the fundamental RF frequency. And, the inclusion of resonant notches in the biasing network is undesirable because it worsens another type of nonlinearity, referred to as “memory effects”. The memory-effect nonlinearities are particularly undesirable because they are difficult to compensate using predistortion techniques which require reasonable computational abilities and consume little power.
In accordance with another technique for addressing the sub-RF distortion signal problem, baseband digital signal processing circuits predict the bias signal corruption that will be caused by the sub-RF distortion signal, then predistort the digital baseband form of the communication signal in a way that will, after upconversion and amplification in the RF power amplifier, compensate for the intermodulation distortion that the sub-RF distortion signal causes. This technique does not rely upon the use of several sub-RF distortion signal bandwidth resonant notches in the biasing network and is effective in reducing the unwanted intermodulation distortion caused by the sub-RF distortion signal. But the bias signal fed to the amplifying device remains less stable in the sub-RF distortion bandwidth than desired.
In addition to linearity requirements set through spectral masks, power-added efficiency (PAE) is another parameter of interest to those who design RF transmitters. PAE is the ratio of the RF output power to the sum of the input RF power and the applied bias-signal power. An amplifier that has low PAE wastes power, which is undesirable in any transmitter, but particularly undesirable in battery-powered transmitters because it necessitates the use of undesirably large batteries and/or undesirably frequent recharges. Conventionally, improvements in PAE have been achieved at the expense of linearity. But envelope-tracking (ET) techniques, envelope elimination and restoration (EER) techniques, and hybrids between the two techniques have shown promise for achieving PAE improvements. When such techniques are combined with conventional digital predistortion techniques, the RF power amplifiers may also achieve modest amounts of linearity.
Generally, envelope tracking (ET), envelope elimination and restoration (EER), and hybrids of the two refer to techniques for biasing an RF power amplifier using a time-varying signal that at least roughly tracks the envelope of the RF communication signal. The goal of such techniques is to provide a bias signal to a bias feed network that maintains the bias voltage and current between the conduction nodes of the RF amplifying device at a level no greater than it needs to be to achieve respectably linear amplification. Conventional transmitters contemplate the use of predistortion to compensate for the nonlinearity that will result from the use of a time-varying bias feed signal rather than a constant signal.
Unfortunately, the sub-RF distortion signal is believed to add a component of bias instability which prevents envelope tracking techniques from achieving desired levels of improvement in PAE.
What is needed is an RF transmitter having an RF power amplifier that achieves both improved PAE and improved linearity by stabilizing the bias applied to RF amplifying devices and by avoiding the excessive use of resonant notches in the biasing network.